Cellular substrates of acoustic and tactile startle responses and their modification by habituation, sensitization, and associative learning will be investigated. Startle is a short latency brainstem and spinal reflex. We have attempted to delineate the neural circuit that mediates acoustic startle and are using this information to determine points within this neural pathway where plasticity occurs so as to affect the final behavioral output. Earlier work showed that short-term habituation of electrically elicited startle occurred when the reflex was elicited through early parts of the startle pathway, whereas sensitization occurred when elicited from caudal points of the pathway. We will now more precisely determine where short-term habituation occurs within the startle pathway and then extend these techniques to analyze long-term habituation and sensitization. A consistent component of the startle reflex in rats is a rapid backward movement of the pinna which displays habituation, pre- pulse inhibition and modification by prior fear conditioning. We will determine the neural pathway that mediates this component of startle using chemical lesions and anterograde and retrograde tracing techniques. The neural pathway that mediates startle elicited by an air-puff will also be investigated. To begin a cellular analysis of startle plasticity, single unit activity will be recorded within the division of the facial motor nucleus that projects to the pinna muscles. We have devised a system which allows us to record single units using conventional glass micropipettes and microdrives in an awake, but restrained rat in a modified stereotaxic apparatus. Rats recorded in this way show acoustically-elicited pinna reflexes that can be quantified and measured simultaneously with single unit activity in the facial motor nucleus. This information should allow us to assess the feasibility of carrying out a cellular analysis of startle plasticity that eventually could be applied to each of the nuclei along the startle pathway.